The reactor cavity walls and the fuel cavity walls in a nuclear power plant have hitherto mainly been cleaned manually with cloths soaked in acetone. Besides being a very time consuming job, it is performed in an unhealthy environment owing both to the radioactivity and the acetone vapour. Attempts have been made to perform this decontamination work by means of rotating brushes and simultaneously sprinkling water over the surface to be cleaned and high pressure washing with the use of chemicals has also been tried, but the purity requirements have not been fulfilled with these methods. A reason for the difficulties is that with the conventional devices it is difficult to achieve the surface pressure against the cavity walls that is required, especially when high pressure washing and brushing are performed simultaneously. Owing to the varying shape of the cavities and the narrow spaces between the cavity wall and projecting structural elements etc., there can only be used a cleaning machine, which hangs from a crane, or a telpher. The cleaning machine must be so constructed that it is easily remote controlled by hand, it must be used in an empty as well as in a waterfilled cavity and it should be easy to decontaminate, e.g. by lowering it into a cleaning bath.
A problem connected with the requirement of having the cleaning machine hanging from wires is that the bearing pressure of the machine against the cavity wall is decreased the deeper the machine descends into the cavity. For cavities with a depth of 10-12 m this is a considerable problem.